I'm thinking for testing it will work out fine. Or should I double my money and go for something like this:

http://www.ebay.co.uk/itm/130936585418

Views welcome

The second one seems to have the wheels more 'centered' front-to-rear. Reminds me more of a Roomba....which does an amazing job of spinning in place. Would probably do a better job of keeping pen centered while turning.

bjpirt

It's a little late, and I'm not sure if you'll still be interested, but I too was keen to build a Turtle robot based on the Arduino for my kids. I've been building it for a while now and thought you might be interested - it's open source (or it will be when I put the designs online shortly) and pretty easy to build. It's WiFi based and battery powered. I've also built a Logo virtual machine that runs on the Arduino which lets you hook up commands to do whatever you want really.

The robot is here: http://mirobot.io

and the Logo VM is here: https://github.com/bjpirt/LogoVM

Give me a shout if you're interested and I'll happily send across the designs (they'll be online in the next week or so anyway)